The historic image of the Milky Way’s central black hole is just the beginning

Last week, a historic scientific achievement was unveiled to the public: the capture of first image of the supermassive black hole Sagittarius A*, which is at the heart of the Milky Way.

Revealing the first real image taken of the Sagittarius A* black hole. Image: European Southern Observatory (ESO)

Now the Event Horizon Telescope (EHT) is ready to take the next steps in observing black holes, making videos that could show gases flowing violently through these mysterious regions of spacetime.


Black holes are constantly rotating as orbits of gas flow around them, at what is called the event horizon. However, motion pictures capable of showing all this turbulence have never been recorded.

The movies produced by repeated images of black holes over months and years are a dream come true for the astronomical scientific community. Researchers hope such movies will show how accretionary disks evolve as gas circulates around them and how magnetic fields inside the disk become entangled and relaxed as they are pulled along by black holes. .

According to Katie Bouman, a computer scientist at the California Institute of Technology (Caltech), there have already been attempts to make a film. “We tried this with data from 2017. We developed algorithms that allowed us to make movies and apply them to the data,” he added. “We saw that while there was something interesting out there, the data we have right now isn’t enough to make anything really confident.”

Scientists therefore need more data before a video is viable. However, capturing this data is time-consuming, and the telescopes that make up the EHT project have other observing programs to complete.

Agile observation could enable the capture of moving images of black holes

depending on the website Space.comto meet the challenge, team engineers are implementing technical improvements so that by 2024 EHT astronomers can modify their observations on and disabled. This capability will allow scientists to use free time at the telescopes for an extended period, rather than an observing campaign that lasts a week or two.

Vincent Fish, an astrophysicist at the Massachusetts Institute of Technology’s Haystack Observatory, describes the approach as nimble observation. “You make your observations, then [os telescópios] can go back and do their other science for the rest of the time,” he said at the US National Science Foundation (NSF) press conference last Thursday.

Although these nimble observations won’t begin until 2024, EHT scientists will need time to process the data into a movie using the imaging techniques described by Bouman.

And the first “movie star” among the black holes will be o Messier 87 (or M87*)which is 7 billion times the mass of the Sun and lies 54 million light-years from Earth at the heart of the Virgo galaxy cluster.

Comparisons between images of Sagittarius A* and M87* black holes. Credit:

Despite its great distance, this black hole appears in the sky at a size similar to Sagittarius A*, due to the fact that it is much larger. The ring of gas pictured around Sagittarius A* could fit within the orbit of Mercury, whose radius is around 58 million kilometers, while the black hole M87* could easily span the orbits of all planets. planets of the solar system. .

Because Sagittarius A* is so much smaller, the changes are happening much faster as gas swirls around the black hole — too quickly for sporadic observation by the EHT to track.

In the case of the M87*, it is so large that changes in its gas ring take weeks or months to manifest, allowing movies to be captured at a greater rate.

Agile observation has other advantages. Sometimes black holes experience an explosion by tearing apart an asteroid or gas cloud that has come too close.

The observation of such explosions requires rapid monitoring, which the EHT has not been able to do so far, given the logistics of organizing the schedule of the telescopes and setting up the necessary equipment. With nimble observation, the EHT will be able to track the movement of a switch if astronomers spot an explosion on M87* or even Sagittarius A*.

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EHT aims to create polarized images of Sagittarius A*

While we don’t have a Sagittarius A* movie any time soon, there’s plenty more to watch in the meantime. The EHT has previously measured the level of polarization in the light of the M87* gas disk, which tells astronomers about the strength and direction of magnetic fields shrouded in the disk, possibly emanating from the black hole itself. And it must also be done in the black hole of our galaxy.

“Our next step will be to create polarized images of Sagittarius A*, so that we can see the magnetic fields near the black hole and see how they are driven. [ao redor] by the black hole itself,” said Michael Johnson, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics, during the NSF videoconference.

The EHT works using very long baseline interferometry, a technique that combines telescopes. The distance between the telescopes, which scientists call the “baseline”, is equivalent to the aperture of a normal telescope.

While seven observatories collaborated to image the M87* black hole, with the addition of the South Pole Telescope, eight observatories were involved in capturing the image of Sagittarius A*.

On Twitter, the EHT team reported that eight telescopes were used when capturing the image of the M87 black hole*

If more telescopes can join the EHT project, the baselines connecting the observatories could increase in number and length. Stretching baselines increases resolution, allowing scientists to see finer details.

In addition, increasing the number of baselines increases the sensitivity of the EHT as well as its number of viewing angles. This is a factor that is displayed in the image of Sagittarius A*, which appears irregular: the bright spots are not hot spots, but rather marking regions where the viewing angles of several pairs of telescopes have coincided, which gives a stronger signal.

Three new telescopes have been added to the EHT since the M87* and Sagittarius A* images were taken: the Greenland Telescope Project (GTP) in Greenland, the IRAM NOEMA Observatory in the French Alps and the Kitt Peak Telescope of 12 meters, in the US state of Arizona.

Since the GTP is so far north, it can only observe the M87* black hole, not Sagittarius A*. On the other hand, the South Pole telescope is not able to see M87*. So, with just 10 telescopes, it will be possible to observe each of these black holes. “Adding new stations will help a lot,” said Ryan Hickox, an astrophysicist at Dartmouth College.

And other black holes in other galaxies? Unfortunately, for now, we may have to settle for two black holes. “One of the challenges is that there really isn’t a black hole that has a large enough event horizon, as projected into the sky, that can be easily captured with the Event Horizon Telescope,” he said. Hickox.

This does not mean that the EHT cannot observe them. The network has already observed jets from some active galaxies, such as quasar 3C273, which is 2.4 billion light-years from Earth and has a central black hole about 880 million solar masses.

These jets can be surprisingly informative, according to Hickox. “There are a lot of really interesting structures in these jets that tell us how particles are accelerated around a black hole, how they interact with the environment after they’re ejected, and how magnetic fields work, and the composition of these particles, and all those things that affect how those jets then influence the very large scale gas around their galaxy.

Many doubts persist about black holes. Do they spin, and if so, how fast? Where do your magnetic fields come from? Do they consume gas in small sips or in homeopathic doses? And how do they affect their immediate environment in their galaxies?

With the release of the Sagittarius A* image and the ability to capture moving images, the answers to some of these questions may be almost at your fingertips.

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